I used to train from Monday to Saturday, 3 hours per day (30 min running, 15-20 min stretching, 1,5 hours of technique, pads or sparring, 30 min calisthenics, 15 min of bike to cool down). Great shape. But I don't think I will be able to maintain this training regime in grad school, obviously... nevertheless I hope to be able to train like this at least 2 or 3 days per week. The rest of the week, I will try to go running as usual (30 min) and train with Tabata intervals (1 Tabata = 8 intervals consisting in 20 seconds of workout - whatever exercise you want - giving your 200%, 10 seconds rest).
I suggest you try Tabata! It doesn't take a lot of time (1 Tabata = 4 min) and it works. It's pretty intense, you don't need more than 4 tabata per day to mantain a great shape. You don't even need a gym.